Injection devices, such as injection pens, are widely used for self administration of liquid drugs by people in need of therapeutic treatment. Many injection devices are capable of repeatedly setting and injecting either a fixed or a variable volume of drug upon operation of respective dose setting and injection mechanisms in the device. Some injection devices are adapted to be loaded with a prefilled drug reservoir containing a volume of drug which is sufficient to provide for a number of injectable doses. When the reservoir is empty, the user replaces it with a new one and the injection device can thus be used again and again. Other injection devices are prefilled when delivered to the user and can only be used until the drug reservoir has been emptied. The various injection devices typically expel the drug by advancing a piston in the reservoir using a motion controlled piston rod.
In order for an injection device to deliver accurate doses it is required that the piston rod is in contact with the piston in the reservoir at any time during the action of the piston rod drive following an activation of the injection mechanism. During transportation and handling of the injection device there is, however, a risk of introducing an undesired slack between the piston rod and the piston which should then be eliminated before an injection, e.g. by carrying out a priming operation. Certain injection devices are provided with a unidirectional coupling mechanism preventing the piston rod from moving proximally, or backwards, in the injection device. As an example, WO 2004/006997 discloses an administering apparatus incorporating a piston rod with two rows of serrated teeth for successive engagement with blocking tongues in a holding part preventing a returning movement of the piston rod in any axial position. Such a non-return arrangement practically eliminates the possibility of slack between the piston rod and the piston once the two components have initially been brought together.
If an injection device accommodating a liquid drug is for any reason exposed to temperatures below the liquid drug's freezing point, the liquid will expand in the reservoir causing a relatively large pressure on the reservoir walls, including the piston. When there is no slack between the piston rod and the piston, the piston rod drive will consequently be exposed to a relatively large backward directed force from the piston via the piston rod. Especially for injection devices employing a unidirectional ratchet and pawl drive mechanism the coupling between the piston rod and the piston rod drive may not be able to withstand such a large force, potentially leading to breakage of vital components in the device, thereby either introducing a risk of malfunction or rendering the device useless.
Some liquid drugs should be stored within certain temperature limits to maintain their desired therapeutic effect. For example, to avoid bacteria growth some liquid drugs should be stored refrigerated. However, accidentally placing the drug container too close to a highly active cooling element may cause the drug to freeze. This issue is particularly relevant for people living in a hot climate. In case the liquid drug itself is not spoiled by the freezing it would be convenient if the encapsulating delivery device was not spoiled either, so the user would avoid wasting the drug anyway. Therefore, it is desirable to provide an injection device which is capable of resisting temperatures below the freezing point of the contained drug, i.e. which is not damaged and rendered useless as a result of the drug expanding in the reservoir.